1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * RCU CPU stall warnings for normal RCU grace periods 4 * 5 * Copyright IBM Corporation, 2019 6 * 7 * Author: Paul E. McKenney <paulmck@linux.ibm.com> 8 */ 9 10 #include <linux/kvm_para.h> 11 12 ////////////////////////////////////////////////////////////////////////////// 13 // 14 // Controlling CPU stall warnings, including delay calculation. 15 16 /* panic() on RCU Stall sysctl. */ 17 int sysctl_panic_on_rcu_stall __read_mostly; 18 int sysctl_max_rcu_stall_to_panic __read_mostly; 19 20 #ifdef CONFIG_PROVE_RCU 21 #define RCU_STALL_DELAY_DELTA (5 * HZ) 22 #else 23 #define RCU_STALL_DELAY_DELTA 0 24 #endif 25 #define RCU_STALL_MIGHT_DIV 8 26 #define RCU_STALL_MIGHT_MIN (2 * HZ) 27 28 int rcu_exp_jiffies_till_stall_check(void) 29 { 30 int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout); 31 int exp_stall_delay_delta = 0; 32 int till_stall_check; 33 34 // Zero says to use rcu_cpu_stall_timeout, but in milliseconds. 35 if (!cpu_stall_timeout) 36 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check()); 37 38 // Limit check must be consistent with the Kconfig limits for 39 // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range. 40 // The minimum clamped value is "2UL", because at least one full 41 // tick has to be guaranteed. 42 till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 21UL * HZ); 43 44 if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout) 45 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check)); 46 47 #ifdef CONFIG_PROVE_RCU 48 /* Add extra ~25% out of till_stall_check. */ 49 exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1; 50 #endif 51 52 return till_stall_check + exp_stall_delay_delta; 53 } 54 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check); 55 56 /* Limit-check stall timeouts specified at boottime and runtime. */ 57 int rcu_jiffies_till_stall_check(void) 58 { 59 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout); 60 61 /* 62 * Limit check must be consistent with the Kconfig limits 63 * for CONFIG_RCU_CPU_STALL_TIMEOUT. 64 */ 65 if (till_stall_check < 3) { 66 WRITE_ONCE(rcu_cpu_stall_timeout, 3); 67 till_stall_check = 3; 68 } else if (till_stall_check > 300) { 69 WRITE_ONCE(rcu_cpu_stall_timeout, 300); 70 till_stall_check = 300; 71 } 72 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; 73 } 74 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check); 75 76 /** 77 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled? 78 * 79 * Returns @true if the current grace period is sufficiently old that 80 * it is reasonable to assume that it might be stalled. This can be 81 * useful when deciding whether to allocate memory to enable RCU-mediated 82 * freeing on the one hand or just invoking synchronize_rcu() on the other. 83 * The latter is preferable when the grace period is stalled. 84 * 85 * Note that sampling of the .gp_start and .gp_seq fields must be done 86 * carefully to avoid false positives at the beginnings and ends of 87 * grace periods. 88 */ 89 bool rcu_gp_might_be_stalled(void) 90 { 91 unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV; 92 unsigned long j = jiffies; 93 94 if (d < RCU_STALL_MIGHT_MIN) 95 d = RCU_STALL_MIGHT_MIN; 96 smp_mb(); // jiffies before .gp_seq to avoid false positives. 97 if (!rcu_gp_in_progress()) 98 return false; 99 // Long delays at this point avoids false positive, but a delay 100 // of ULONG_MAX/4 jiffies voids your no-false-positive warranty. 101 smp_mb(); // .gp_seq before second .gp_start 102 // And ditto here. 103 return !time_before(j, READ_ONCE(rcu_state.gp_start) + d); 104 } 105 106 /* Don't do RCU CPU stall warnings during long sysrq printouts. */ 107 void rcu_sysrq_start(void) 108 { 109 if (!rcu_cpu_stall_suppress) 110 rcu_cpu_stall_suppress = 2; 111 } 112 113 void rcu_sysrq_end(void) 114 { 115 if (rcu_cpu_stall_suppress == 2) 116 rcu_cpu_stall_suppress = 0; 117 } 118 119 /* Don't print RCU CPU stall warnings during a kernel panic. */ 120 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) 121 { 122 rcu_cpu_stall_suppress = 1; 123 return NOTIFY_DONE; 124 } 125 126 static struct notifier_block rcu_panic_block = { 127 .notifier_call = rcu_panic, 128 }; 129 130 static int __init check_cpu_stall_init(void) 131 { 132 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); 133 return 0; 134 } 135 early_initcall(check_cpu_stall_init); 136 137 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */ 138 static void panic_on_rcu_stall(void) 139 { 140 static int cpu_stall; 141 142 if (++cpu_stall < sysctl_max_rcu_stall_to_panic) 143 return; 144 145 if (sysctl_panic_on_rcu_stall) 146 panic("RCU Stall\n"); 147 } 148 149 /** 150 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period 151 * 152 * The caller must disable hard irqs. 153 */ 154 void rcu_cpu_stall_reset(void) 155 { 156 WRITE_ONCE(rcu_state.jiffies_stall, 157 jiffies + rcu_jiffies_till_stall_check()); 158 } 159 160 ////////////////////////////////////////////////////////////////////////////// 161 // 162 // Interaction with RCU grace periods 163 164 /* Start of new grace period, so record stall time (and forcing times). */ 165 static void record_gp_stall_check_time(void) 166 { 167 unsigned long j = jiffies; 168 unsigned long j1; 169 170 WRITE_ONCE(rcu_state.gp_start, j); 171 j1 = rcu_jiffies_till_stall_check(); 172 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq. 173 WRITE_ONCE(rcu_state.jiffies_stall, j + j1); 174 rcu_state.jiffies_resched = j + j1 / 2; 175 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs); 176 } 177 178 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */ 179 static void zero_cpu_stall_ticks(struct rcu_data *rdp) 180 { 181 rdp->ticks_this_gp = 0; 182 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id()); 183 WRITE_ONCE(rdp->last_fqs_resched, jiffies); 184 } 185 186 /* 187 * If too much time has passed in the current grace period, and if 188 * so configured, go kick the relevant kthreads. 189 */ 190 static void rcu_stall_kick_kthreads(void) 191 { 192 unsigned long j; 193 194 if (!READ_ONCE(rcu_kick_kthreads)) 195 return; 196 j = READ_ONCE(rcu_state.jiffies_kick_kthreads); 197 if (time_after(jiffies, j) && rcu_state.gp_kthread && 198 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) { 199 WARN_ONCE(1, "Kicking %s grace-period kthread\n", 200 rcu_state.name); 201 rcu_ftrace_dump(DUMP_ALL); 202 wake_up_process(rcu_state.gp_kthread); 203 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ); 204 } 205 } 206 207 /* 208 * Handler for the irq_work request posted about halfway into the RCU CPU 209 * stall timeout, and used to detect excessive irq disabling. Set state 210 * appropriately, but just complain if there is unexpected state on entry. 211 */ 212 static void rcu_iw_handler(struct irq_work *iwp) 213 { 214 struct rcu_data *rdp; 215 struct rcu_node *rnp; 216 217 rdp = container_of(iwp, struct rcu_data, rcu_iw); 218 rnp = rdp->mynode; 219 raw_spin_lock_rcu_node(rnp); 220 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) { 221 rdp->rcu_iw_gp_seq = rnp->gp_seq; 222 rdp->rcu_iw_pending = false; 223 } 224 raw_spin_unlock_rcu_node(rnp); 225 } 226 227 ////////////////////////////////////////////////////////////////////////////// 228 // 229 // Printing RCU CPU stall warnings 230 231 #ifdef CONFIG_PREEMPT_RCU 232 233 /* 234 * Dump detailed information for all tasks blocking the current RCU 235 * grace period on the specified rcu_node structure. 236 */ 237 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) 238 { 239 unsigned long flags; 240 struct task_struct *t; 241 242 raw_spin_lock_irqsave_rcu_node(rnp, flags); 243 if (!rcu_preempt_blocked_readers_cgp(rnp)) { 244 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 245 return; 246 } 247 t = list_entry(rnp->gp_tasks->prev, 248 struct task_struct, rcu_node_entry); 249 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) { 250 /* 251 * We could be printing a lot while holding a spinlock. 252 * Avoid triggering hard lockup. 253 */ 254 touch_nmi_watchdog(); 255 sched_show_task(t); 256 } 257 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 258 } 259 260 // Communicate task state back to the RCU CPU stall warning request. 261 struct rcu_stall_chk_rdr { 262 int nesting; 263 union rcu_special rs; 264 bool on_blkd_list; 265 }; 266 267 /* 268 * Report out the state of a not-running task that is stalling the 269 * current RCU grace period. 270 */ 271 static int check_slow_task(struct task_struct *t, void *arg) 272 { 273 struct rcu_stall_chk_rdr *rscrp = arg; 274 275 if (task_curr(t)) 276 return -EBUSY; // It is running, so decline to inspect it. 277 rscrp->nesting = t->rcu_read_lock_nesting; 278 rscrp->rs = t->rcu_read_unlock_special; 279 rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry); 280 return 0; 281 } 282 283 /* 284 * Scan the current list of tasks blocked within RCU read-side critical 285 * sections, printing out the tid of each of the first few of them. 286 */ 287 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) 288 __releases(rnp->lock) 289 { 290 int i = 0; 291 int ndetected = 0; 292 struct rcu_stall_chk_rdr rscr; 293 struct task_struct *t; 294 struct task_struct *ts[8]; 295 296 lockdep_assert_irqs_disabled(); 297 if (!rcu_preempt_blocked_readers_cgp(rnp)) { 298 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 299 return 0; 300 } 301 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):", 302 rnp->level, rnp->grplo, rnp->grphi); 303 t = list_entry(rnp->gp_tasks->prev, 304 struct task_struct, rcu_node_entry); 305 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) { 306 get_task_struct(t); 307 ts[i++] = t; 308 if (i >= ARRAY_SIZE(ts)) 309 break; 310 } 311 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 312 while (i) { 313 t = ts[--i]; 314 if (task_call_func(t, check_slow_task, &rscr)) 315 pr_cont(" P%d", t->pid); 316 else 317 pr_cont(" P%d/%d:%c%c%c%c", 318 t->pid, rscr.nesting, 319 ".b"[rscr.rs.b.blocked], 320 ".q"[rscr.rs.b.need_qs], 321 ".e"[rscr.rs.b.exp_hint], 322 ".l"[rscr.on_blkd_list]); 323 lockdep_assert_irqs_disabled(); 324 put_task_struct(t); 325 ndetected++; 326 } 327 pr_cont("\n"); 328 return ndetected; 329 } 330 331 #else /* #ifdef CONFIG_PREEMPT_RCU */ 332 333 /* 334 * Because preemptible RCU does not exist, we never have to check for 335 * tasks blocked within RCU read-side critical sections. 336 */ 337 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) 338 { 339 } 340 341 /* 342 * Because preemptible RCU does not exist, we never have to check for 343 * tasks blocked within RCU read-side critical sections. 344 */ 345 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) 346 __releases(rnp->lock) 347 { 348 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 349 return 0; 350 } 351 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */ 352 353 /* 354 * Dump stacks of all tasks running on stalled CPUs. First try using 355 * NMIs, but fall back to manual remote stack tracing on architectures 356 * that don't support NMI-based stack dumps. The NMI-triggered stack 357 * traces are more accurate because they are printed by the target CPU. 358 */ 359 static void rcu_dump_cpu_stacks(void) 360 { 361 int cpu; 362 unsigned long flags; 363 struct rcu_node *rnp; 364 365 rcu_for_each_leaf_node(rnp) { 366 raw_spin_lock_irqsave_rcu_node(rnp, flags); 367 for_each_leaf_node_possible_cpu(rnp, cpu) 368 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) { 369 if (cpu_is_offline(cpu)) 370 pr_err("Offline CPU %d blocking current GP.\n", cpu); 371 else if (!trigger_single_cpu_backtrace(cpu)) 372 dump_cpu_task(cpu); 373 } 374 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 375 } 376 } 377 378 static const char * const gp_state_names[] = { 379 [RCU_GP_IDLE] = "RCU_GP_IDLE", 380 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS", 381 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS", 382 [RCU_GP_ONOFF] = "RCU_GP_ONOFF", 383 [RCU_GP_INIT] = "RCU_GP_INIT", 384 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS", 385 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS", 386 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP", 387 [RCU_GP_CLEANED] = "RCU_GP_CLEANED", 388 }; 389 390 /* 391 * Convert a ->gp_state value to a character string. 392 */ 393 static const char *gp_state_getname(short gs) 394 { 395 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names)) 396 return "???"; 397 return gp_state_names[gs]; 398 } 399 400 /* Is the RCU grace-period kthread being starved of CPU time? */ 401 static bool rcu_is_gp_kthread_starving(unsigned long *jp) 402 { 403 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity); 404 405 if (jp) 406 *jp = j; 407 return j > 2 * HZ; 408 } 409 410 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp) 411 { 412 int cpu; 413 struct task_struct *rcuc; 414 unsigned long j; 415 416 rcuc = rdp->rcu_cpu_kthread_task; 417 if (!rcuc) 418 return false; 419 420 cpu = task_cpu(rcuc); 421 if (cpu_is_offline(cpu) || idle_cpu(cpu)) 422 return false; 423 424 j = jiffies - READ_ONCE(rdp->rcuc_activity); 425 426 if (jp) 427 *jp = j; 428 return j > 2 * HZ; 429 } 430 431 /* 432 * Print out diagnostic information for the specified stalled CPU. 433 * 434 * If the specified CPU is aware of the current RCU grace period, then 435 * print the number of scheduling clock interrupts the CPU has taken 436 * during the time that it has been aware. Otherwise, print the number 437 * of RCU grace periods that this CPU is ignorant of, for example, "1" 438 * if the CPU was aware of the previous grace period. 439 * 440 * Also print out idle info. 441 */ 442 static void print_cpu_stall_info(int cpu) 443 { 444 unsigned long delta; 445 bool falsepositive; 446 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); 447 char *ticks_title; 448 unsigned long ticks_value; 449 bool rcuc_starved; 450 unsigned long j; 451 char buf[32]; 452 453 /* 454 * We could be printing a lot while holding a spinlock. Avoid 455 * triggering hard lockup. 456 */ 457 touch_nmi_watchdog(); 458 459 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq); 460 if (ticks_value) { 461 ticks_title = "GPs behind"; 462 } else { 463 ticks_title = "ticks this GP"; 464 ticks_value = rdp->ticks_this_gp; 465 } 466 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq); 467 falsepositive = rcu_is_gp_kthread_starving(NULL) && 468 rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu)); 469 rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j); 470 if (rcuc_starved) 471 sprintf(buf, " rcuc=%ld jiffies(starved)", j); 472 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n", 473 cpu, 474 "O."[!!cpu_online(cpu)], 475 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)], 476 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)], 477 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' : 478 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' : 479 "!."[!delta], 480 ticks_value, ticks_title, 481 rcu_dynticks_snap(cpu) & 0xffff, 482 ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu), 483 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu), 484 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart, 485 rcuc_starved ? buf : "", 486 falsepositive ? " (false positive?)" : ""); 487 } 488 489 /* Complain about starvation of grace-period kthread. */ 490 static void rcu_check_gp_kthread_starvation(void) 491 { 492 int cpu; 493 struct task_struct *gpk = rcu_state.gp_kthread; 494 unsigned long j; 495 496 if (rcu_is_gp_kthread_starving(&j)) { 497 cpu = gpk ? task_cpu(gpk) : -1; 498 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n", 499 rcu_state.name, j, 500 (long)rcu_seq_current(&rcu_state.gp_seq), 501 data_race(READ_ONCE(rcu_state.gp_flags)), 502 gp_state_getname(rcu_state.gp_state), 503 data_race(READ_ONCE(rcu_state.gp_state)), 504 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu); 505 if (gpk) { 506 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name); 507 pr_err("RCU grace-period kthread stack dump:\n"); 508 sched_show_task(gpk); 509 if (cpu >= 0) { 510 if (cpu_is_offline(cpu)) { 511 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu); 512 } else { 513 pr_err("Stack dump where RCU GP kthread last ran:\n"); 514 if (!trigger_single_cpu_backtrace(cpu)) 515 dump_cpu_task(cpu); 516 } 517 } 518 wake_up_process(gpk); 519 } 520 } 521 } 522 523 /* Complain about missing wakeups from expired fqs wait timer */ 524 static void rcu_check_gp_kthread_expired_fqs_timer(void) 525 { 526 struct task_struct *gpk = rcu_state.gp_kthread; 527 short gp_state; 528 unsigned long jiffies_fqs; 529 int cpu; 530 531 /* 532 * Order reads of .gp_state and .jiffies_force_qs. 533 * Matching smp_wmb() is present in rcu_gp_fqs_loop(). 534 */ 535 gp_state = smp_load_acquire(&rcu_state.gp_state); 536 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs); 537 538 if (gp_state == RCU_GP_WAIT_FQS && 539 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) && 540 gpk && !READ_ONCE(gpk->on_rq)) { 541 cpu = task_cpu(gpk); 542 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n", 543 rcu_state.name, (jiffies - jiffies_fqs), 544 (long)rcu_seq_current(&rcu_state.gp_seq), 545 data_race(rcu_state.gp_flags), 546 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS, 547 data_race(READ_ONCE(gpk->__state))); 548 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n", 549 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu)); 550 } 551 } 552 553 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) 554 { 555 int cpu; 556 unsigned long flags; 557 unsigned long gpa; 558 unsigned long j; 559 int ndetected = 0; 560 struct rcu_node *rnp; 561 long totqlen = 0; 562 563 lockdep_assert_irqs_disabled(); 564 565 /* Kick and suppress, if so configured. */ 566 rcu_stall_kick_kthreads(); 567 if (rcu_stall_is_suppressed()) 568 return; 569 570 /* 571 * OK, time to rat on our buddy... 572 * See Documentation/RCU/stallwarn.rst for info on how to debug 573 * RCU CPU stall warnings. 574 */ 575 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected")); 576 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name); 577 rcu_for_each_leaf_node(rnp) { 578 raw_spin_lock_irqsave_rcu_node(rnp, flags); 579 if (rnp->qsmask != 0) { 580 for_each_leaf_node_possible_cpu(rnp, cpu) 581 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) { 582 print_cpu_stall_info(cpu); 583 ndetected++; 584 } 585 } 586 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock. 587 lockdep_assert_irqs_disabled(); 588 } 589 590 for_each_possible_cpu(cpu) 591 totqlen += rcu_get_n_cbs_cpu(cpu); 592 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n", 593 smp_processor_id(), (long)(jiffies - gps), 594 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus); 595 if (ndetected) { 596 rcu_dump_cpu_stacks(); 597 598 /* Complain about tasks blocking the grace period. */ 599 rcu_for_each_leaf_node(rnp) 600 rcu_print_detail_task_stall_rnp(rnp); 601 } else { 602 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) { 603 pr_err("INFO: Stall ended before state dump start\n"); 604 } else { 605 j = jiffies; 606 gpa = data_race(READ_ONCE(rcu_state.gp_activity)); 607 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n", 608 rcu_state.name, j - gpa, j, gpa, 609 data_race(READ_ONCE(jiffies_till_next_fqs)), 610 data_race(READ_ONCE(rcu_get_root()->qsmask))); 611 } 612 } 613 /* Rewrite if needed in case of slow consoles. */ 614 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall))) 615 WRITE_ONCE(rcu_state.jiffies_stall, 616 jiffies + 3 * rcu_jiffies_till_stall_check() + 3); 617 618 rcu_check_gp_kthread_expired_fqs_timer(); 619 rcu_check_gp_kthread_starvation(); 620 621 panic_on_rcu_stall(); 622 623 rcu_force_quiescent_state(); /* Kick them all. */ 624 } 625 626 static void print_cpu_stall(unsigned long gps) 627 { 628 int cpu; 629 unsigned long flags; 630 struct rcu_data *rdp = this_cpu_ptr(&rcu_data); 631 struct rcu_node *rnp = rcu_get_root(); 632 long totqlen = 0; 633 634 lockdep_assert_irqs_disabled(); 635 636 /* Kick and suppress, if so configured. */ 637 rcu_stall_kick_kthreads(); 638 if (rcu_stall_is_suppressed()) 639 return; 640 641 /* 642 * OK, time to rat on ourselves... 643 * See Documentation/RCU/stallwarn.rst for info on how to debug 644 * RCU CPU stall warnings. 645 */ 646 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected")); 647 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name); 648 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags); 649 print_cpu_stall_info(smp_processor_id()); 650 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags); 651 for_each_possible_cpu(cpu) 652 totqlen += rcu_get_n_cbs_cpu(cpu); 653 pr_cont("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n", 654 jiffies - gps, 655 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus); 656 657 rcu_check_gp_kthread_expired_fqs_timer(); 658 rcu_check_gp_kthread_starvation(); 659 660 rcu_dump_cpu_stacks(); 661 662 raw_spin_lock_irqsave_rcu_node(rnp, flags); 663 /* Rewrite if needed in case of slow consoles. */ 664 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall))) 665 WRITE_ONCE(rcu_state.jiffies_stall, 666 jiffies + 3 * rcu_jiffies_till_stall_check() + 3); 667 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 668 669 panic_on_rcu_stall(); 670 671 /* 672 * Attempt to revive the RCU machinery by forcing a context switch. 673 * 674 * A context switch would normally allow the RCU state machine to make 675 * progress and it could be we're stuck in kernel space without context 676 * switches for an entirely unreasonable amount of time. 677 */ 678 set_tsk_need_resched(current); 679 set_preempt_need_resched(); 680 } 681 682 static void check_cpu_stall(struct rcu_data *rdp) 683 { 684 bool didstall = false; 685 unsigned long gs1; 686 unsigned long gs2; 687 unsigned long gps; 688 unsigned long j; 689 unsigned long jn; 690 unsigned long js; 691 struct rcu_node *rnp; 692 693 lockdep_assert_irqs_disabled(); 694 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) || 695 !rcu_gp_in_progress()) 696 return; 697 rcu_stall_kick_kthreads(); 698 j = jiffies; 699 700 /* 701 * Lots of memory barriers to reject false positives. 702 * 703 * The idea is to pick up rcu_state.gp_seq, then 704 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally 705 * another copy of rcu_state.gp_seq. These values are updated in 706 * the opposite order with memory barriers (or equivalent) during 707 * grace-period initialization and cleanup. Now, a false positive 708 * can occur if we get an new value of rcu_state.gp_start and a old 709 * value of rcu_state.jiffies_stall. But given the memory barriers, 710 * the only way that this can happen is if one grace period ends 711 * and another starts between these two fetches. This is detected 712 * by comparing the second fetch of rcu_state.gp_seq with the 713 * previous fetch from rcu_state.gp_seq. 714 * 715 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall, 716 * and rcu_state.gp_start suffice to forestall false positives. 717 */ 718 gs1 = READ_ONCE(rcu_state.gp_seq); 719 smp_rmb(); /* Pick up ->gp_seq first... */ 720 js = READ_ONCE(rcu_state.jiffies_stall); 721 smp_rmb(); /* ...then ->jiffies_stall before the rest... */ 722 gps = READ_ONCE(rcu_state.gp_start); 723 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */ 724 gs2 = READ_ONCE(rcu_state.gp_seq); 725 if (gs1 != gs2 || 726 ULONG_CMP_LT(j, js) || 727 ULONG_CMP_GE(gps, js)) 728 return; /* No stall or GP completed since entering function. */ 729 rnp = rdp->mynode; 730 jn = jiffies + ULONG_MAX / 2; 731 if (rcu_gp_in_progress() && 732 (READ_ONCE(rnp->qsmask) & rdp->grpmask) && 733 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { 734 735 /* 736 * If a virtual machine is stopped by the host it can look to 737 * the watchdog like an RCU stall. Check to see if the host 738 * stopped the vm. 739 */ 740 if (kvm_check_and_clear_guest_paused()) 741 return; 742 743 /* We haven't checked in, so go dump stack. */ 744 print_cpu_stall(gps); 745 if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) 746 rcu_ftrace_dump(DUMP_ALL); 747 didstall = true; 748 749 } else if (rcu_gp_in_progress() && 750 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) && 751 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { 752 753 /* 754 * If a virtual machine is stopped by the host it can look to 755 * the watchdog like an RCU stall. Check to see if the host 756 * stopped the vm. 757 */ 758 if (kvm_check_and_clear_guest_paused()) 759 return; 760 761 /* They had a few time units to dump stack, so complain. */ 762 print_other_cpu_stall(gs2, gps); 763 if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) 764 rcu_ftrace_dump(DUMP_ALL); 765 didstall = true; 766 } 767 if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) { 768 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3; 769 WRITE_ONCE(rcu_state.jiffies_stall, jn); 770 } 771 } 772 773 ////////////////////////////////////////////////////////////////////////////// 774 // 775 // RCU forward-progress mechanisms, including of callback invocation. 776 777 778 /* 779 * Check to see if a failure to end RCU priority inversion was due to 780 * a CPU not passing through a quiescent state. When this happens, there 781 * is nothing that RCU priority boosting can do to help, so we shouldn't 782 * count this as an RCU priority boosting failure. A return of true says 783 * RCU priority boosting is to blame, and false says otherwise. If false 784 * is returned, the first of the CPUs to blame is stored through cpup. 785 * If there was no CPU blocking the current grace period, but also nothing 786 * in need of being boosted, *cpup is set to -1. This can happen in case 787 * of vCPU preemption while the last CPU is reporting its quiscent state, 788 * for example. 789 * 790 * If cpup is NULL, then a lockless quick check is carried out, suitable 791 * for high-rate usage. On the other hand, if cpup is non-NULL, each 792 * rcu_node structure's ->lock is acquired, ruling out high-rate usage. 793 */ 794 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup) 795 { 796 bool atb = false; 797 int cpu; 798 unsigned long flags; 799 struct rcu_node *rnp; 800 801 rcu_for_each_leaf_node(rnp) { 802 if (!cpup) { 803 if (data_race(READ_ONCE(rnp->qsmask))) { 804 return false; 805 } else { 806 if (READ_ONCE(rnp->gp_tasks)) 807 atb = true; 808 continue; 809 } 810 } 811 *cpup = -1; 812 raw_spin_lock_irqsave_rcu_node(rnp, flags); 813 if (rnp->gp_tasks) 814 atb = true; 815 if (!rnp->qsmask) { 816 // No CPUs without quiescent states for this rnp. 817 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 818 continue; 819 } 820 // Find the first holdout CPU. 821 for_each_leaf_node_possible_cpu(rnp, cpu) { 822 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) { 823 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 824 *cpup = cpu; 825 return false; 826 } 827 } 828 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 829 } 830 // Can't blame CPUs, so must blame RCU priority boosting. 831 return atb; 832 } 833 EXPORT_SYMBOL_GPL(rcu_check_boost_fail); 834 835 /* 836 * Show the state of the grace-period kthreads. 837 */ 838 void show_rcu_gp_kthreads(void) 839 { 840 unsigned long cbs = 0; 841 int cpu; 842 unsigned long j; 843 unsigned long ja; 844 unsigned long jr; 845 unsigned long js; 846 unsigned long jw; 847 struct rcu_data *rdp; 848 struct rcu_node *rnp; 849 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread); 850 851 j = jiffies; 852 ja = j - data_race(READ_ONCE(rcu_state.gp_activity)); 853 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity)); 854 js = j - data_race(READ_ONCE(rcu_state.gp_start)); 855 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time)); 856 pr_info("%s: wait state: %s(%d) ->state: %#x ->rt_priority %u delta ->gp_start %lu ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_max %lu ->gp_flags %#x\n", 857 rcu_state.name, gp_state_getname(rcu_state.gp_state), 858 data_race(READ_ONCE(rcu_state.gp_state)), 859 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU, 860 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)), 861 (long)data_race(READ_ONCE(rcu_state.gp_seq)), 862 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)), 863 data_race(READ_ONCE(rcu_state.gp_max)), 864 data_race(READ_ONCE(rcu_state.gp_flags))); 865 rcu_for_each_node_breadth_first(rnp) { 866 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) && 867 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) && 868 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks))) 869 continue; 870 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n", 871 rnp->grplo, rnp->grphi, 872 (long)data_race(READ_ONCE(rnp->gp_seq)), 873 (long)data_race(READ_ONCE(rnp->gp_seq_needed)), 874 data_race(READ_ONCE(rnp->qsmask)), 875 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))], 876 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))], 877 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))], 878 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))], 879 data_race(READ_ONCE(rnp->n_boosts))); 880 if (!rcu_is_leaf_node(rnp)) 881 continue; 882 for_each_leaf_node_possible_cpu(rnp, cpu) { 883 rdp = per_cpu_ptr(&rcu_data, cpu); 884 if (READ_ONCE(rdp->gpwrap) || 885 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), 886 READ_ONCE(rdp->gp_seq_needed))) 887 continue; 888 pr_info("\tcpu %d ->gp_seq_needed %ld\n", 889 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed))); 890 } 891 } 892 for_each_possible_cpu(cpu) { 893 rdp = per_cpu_ptr(&rcu_data, cpu); 894 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked)); 895 if (rcu_segcblist_is_offloaded(&rdp->cblist)) 896 show_rcu_nocb_state(rdp); 897 } 898 pr_info("RCU callbacks invoked since boot: %lu\n", cbs); 899 show_rcu_tasks_gp_kthreads(); 900 } 901 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads); 902 903 /* 904 * This function checks for grace-period requests that fail to motivate 905 * RCU to come out of its idle mode. 906 */ 907 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp, 908 const unsigned long gpssdelay) 909 { 910 unsigned long flags; 911 unsigned long j; 912 struct rcu_node *rnp_root = rcu_get_root(); 913 static atomic_t warned = ATOMIC_INIT(0); 914 915 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() || 916 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), 917 READ_ONCE(rnp_root->gp_seq_needed)) || 918 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread. 919 return; 920 j = jiffies; /* Expensive access, and in common case don't get here. */ 921 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || 922 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || 923 atomic_read(&warned)) 924 return; 925 926 raw_spin_lock_irqsave_rcu_node(rnp, flags); 927 j = jiffies; 928 if (rcu_gp_in_progress() || 929 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), 930 READ_ONCE(rnp_root->gp_seq_needed)) || 931 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || 932 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || 933 atomic_read(&warned)) { 934 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 935 return; 936 } 937 /* Hold onto the leaf lock to make others see warned==1. */ 938 939 if (rnp_root != rnp) 940 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */ 941 j = jiffies; 942 if (rcu_gp_in_progress() || 943 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), 944 READ_ONCE(rnp_root->gp_seq_needed)) || 945 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) || 946 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) || 947 atomic_xchg(&warned, 1)) { 948 if (rnp_root != rnp) 949 /* irqs remain disabled. */ 950 raw_spin_unlock_rcu_node(rnp_root); 951 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 952 return; 953 } 954 WARN_ON(1); 955 if (rnp_root != rnp) 956 raw_spin_unlock_rcu_node(rnp_root); 957 raw_spin_unlock_irqrestore_rcu_node(rnp, flags); 958 show_rcu_gp_kthreads(); 959 } 960 961 /* 962 * Do a forward-progress check for rcutorture. This is normally invoked 963 * due to an OOM event. The argument "j" gives the time period during 964 * which rcutorture would like progress to have been made. 965 */ 966 void rcu_fwd_progress_check(unsigned long j) 967 { 968 unsigned long cbs; 969 int cpu; 970 unsigned long max_cbs = 0; 971 int max_cpu = -1; 972 struct rcu_data *rdp; 973 974 if (rcu_gp_in_progress()) { 975 pr_info("%s: GP age %lu jiffies\n", 976 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start))); 977 show_rcu_gp_kthreads(); 978 } else { 979 pr_info("%s: Last GP end %lu jiffies ago\n", 980 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end))); 981 preempt_disable(); 982 rdp = this_cpu_ptr(&rcu_data); 983 rcu_check_gp_start_stall(rdp->mynode, rdp, j); 984 preempt_enable(); 985 } 986 for_each_possible_cpu(cpu) { 987 cbs = rcu_get_n_cbs_cpu(cpu); 988 if (!cbs) 989 continue; 990 if (max_cpu < 0) 991 pr_info("%s: callbacks", __func__); 992 pr_cont(" %d: %lu", cpu, cbs); 993 if (cbs <= max_cbs) 994 continue; 995 max_cbs = cbs; 996 max_cpu = cpu; 997 } 998 if (max_cpu >= 0) 999 pr_cont("\n"); 1000 } 1001 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check); 1002 1003 /* Commandeer a sysrq key to dump RCU's tree. */ 1004 static bool sysrq_rcu; 1005 module_param(sysrq_rcu, bool, 0444); 1006 1007 /* Dump grace-period-request information due to commandeered sysrq. */ 1008 static void sysrq_show_rcu(int key) 1009 { 1010 show_rcu_gp_kthreads(); 1011 } 1012 1013 static const struct sysrq_key_op sysrq_rcudump_op = { 1014 .handler = sysrq_show_rcu, 1015 .help_msg = "show-rcu(y)", 1016 .action_msg = "Show RCU tree", 1017 .enable_mask = SYSRQ_ENABLE_DUMP, 1018 }; 1019 1020 static int __init rcu_sysrq_init(void) 1021 { 1022 if (sysrq_rcu) 1023 return register_sysrq_key('y', &sysrq_rcudump_op); 1024 return 0; 1025 } 1026 early_initcall(rcu_sysrq_init); 1027